The vacuum interrupter has a structure in which a fixed electrode and a movable electrode are disposed oppositely on a same axis in an insulated container with its interior kept at a high vacuum. When carrying a current, the fixed electrode and the movable electrode are in contact with each other. When an overcurrent or short-circuit-current occurs, these electrodes are instantaneously opened to interrupt the current.
Contact material used for the contacting portions of the fixed electrode and the movable electrode of the vacuum interrupter is mainly required to have a current-interruption performance and a voltage-withstand performance at an instance of opening electrodes. These performances required for the contact material are properties contradictory to each other; therefore, it is difficult to produce the contact material using a material consisting of a single element. Thus, conventional contact materials have been produced using a material which is a combination of two or more elements.
For example, for the voltage-resistance material, a contact material such as a Cu—W type or a Cu—Cr type is generally used in which copper (Cu) being a high-conductivity material, and tungsten (W) or chromium (Cr) are used. In some cases, for a contact material of a vacuum interrupter requiring a low surge characteristic, a contact material such as a Cu—WC type or an Ag—WC type is generally used in which tungsten carbide (WC) being an electron emission constituent is dispersed in copper (Cu) or silver (Ag) being a high-conductivity material, in order to extend current-break time.
As a method for producing these contact materials, an infiltration method described below is used. First, raw material powder of a voltage-resistance material is molded and sintered into a porous body. Then, an infiltrant composed of Cu, Ag or the like is placed on one side of the porous body and heated at the infiltrant melting point or higher. The melted infiltrant permeates (infiltrates) pores in the porous body. A material plate resultantly obtained for the contact is machined into a required contact shape, to obtain the contact. After machined into the contact shape, the contact is brazed to a copper rod which serves as a conductor for carrying a current. In a case where the ratio of a voltage-resistance material constituent having low wettability to the brazing material is large at the contact surface, the brazing may be insufficient, which sometimes causes the contact to fall off or have a small contact area between the copper rod and the contact.
To cope with this problem, there is a method in which tungsten powder is compression-molded into a porous body (refer to Patent Document 1). In this method, a lower punch to be used has been specially devised to form a recess on the molded medium side in contact with the infiltrant. The porous body is put on the infiltrant; and the infiltrant is heated to infiltrate the porous body, whereby the infiltrant metal remains in the recess. After a finishing process for the sintered medium, the sintered medium is joined to a base metal with a brazing material via a remaining infiltrant layer. Even when the contact part contains a hard-to-join material, this method brings no brazing problem because the infiltrant and the base metal are joined.